In a number of diverse industries, including microelectronics, biotechnology and microsystems, it is important to produce high resolution patterned structures on substrates. For example, high resolution patterned structures are necessary to produce integrated circuits. Presently, photolithography is the most commonly used process to produce these patterned structures on substrates.
Photolithography techniques involve exposing a photoresist to an optical pattern and using chemicals to etch either the exposed or unexposed portions of the photoresist to produce the patterned structure on the substrate. Because photolithography is only limited by the wavelength of light used to produce the optical pattern, it allows for the production of devices with very small features.
While conventional UV lithography offers unparalleled resolution for device fabrication, photolithography becomes increasingly complex and costly when sub-micron features are required. It is also not readily adaptable to the patterning of curved substrates or patterning of films consisting of materials which are not UV compatible. Accordingly, there is growing interest in alternative, less costly and more rapid patterning techniques for construction of micro-optic, photonic and optoelectronic devices. It has been shown, for example, that microcontact printing, micromolding, microembossing or nanoimprinting can fabricate components for MEMS devices more cheaply. (See, e.g., L. J. Guo, Adv. Mat. 19, 495 (2007) & E. Menard et al., Chem. Rev. 107, 1117 (2007), the disclosure of which are incorporated herein by reference.) Letterpress techniques have also been used to fabricate polymeric masks for resist-free printing of amorphous silicon thin film transistors whose performance is equivalent to those fabricated by conventional means. (See, e.g., S. M. Miller, et al., J. Vac. Sci. Tech. B 20, 2320 (2002) & S. M. Miller, et al., Appl. Phys. Lett. 83 (15), 3207 (2003), the disclosures of which are incorporated herein by reference.) Equally promising are techniques for the construction of microscale components by non-contact means such as electrohydrodynamic ink-jetting, whereby small volumes are jetted onto selective sites of a target substrate. (See, e.g., J. Park et al., Nature Materials 6, 782 (2007), the disclosure of which is incorporated herein by reference.) However, those techniques that depend on structure formation by deposition of material, like ink jetting, are inherently 2D in that the object or device to be patterned is constructed by adding and subtracting material layer by layer to achieve the final desired shape. These techniques therefore required multiple process steps. Those techniques which depend on molding technologies, by contrast, require a 3D mold whose shape must be pressed firmly into the film to be patterned. Necessary contact of the mold with the liquefied film leads to difficulties with adhesion and film detachment when removing the mold. Thermocapillary lithography, however, allows for controlled and true 3D construction of small micro and nanoscale patterns by non-contact means and in a minimal number of process steps.
The interest in inexpensive fabrication of either single structures or devices or large area pattern arrays has led a number of groups to investigate the use of natural fluid instabilities for non-contact patterning of molten polymer films. Specifically, during the past decade, three independent groups have reported experiments in which an ultrathin molten polymer film sandwiched in between two rigid substrates, as shown in FIG. 1, and subject to a vertical thermal gradient of order 106-108° C./cm, can undergo spontaneous formation of nanopillars, nanospirals, or other 3D protrusions after several minutes or hours depending on the value of the thermal gradient, the viscosity of the molten film and various other geometric and material parameters. In all reported experiments, the structures were allowed to grow and make contact with the cooler target substrate after which the thermal gradient was removed. Upon removal of this gradient, the polymeric structures solidified in place after which the top surface was removed. (See, e.g., S. Y. Chou, et al., Appl. Phys. Lett, 75, 1004 (1999); S. Y. Chou and L. Zhuang, J. Vac. Sci. Technol. B, 17(6), 3197 (1999); E. Schäffer, PhD Thesis, Univ. of Konstanz, Germany (2001); E. Schäffer, et al., Adv. Mater. 15(6), 514 (2003); E. Schäffer, et al., Europhys. Lett., 60(2), 255 (2002); E. Schäffer, et al., Macromolecules 36, 1645 (2003); and J. Peng, et al., Polymer 45, 8013 (2004), the disclosures of each of which are incorporated herein by reference.)
Schäffer et al. not only conducted experiments, but postulated in 2001, and in subsequent papers, that the cause of the formation of these pillared arrays was due to a type of fluid instability associated with pressure buildup from interfacial reflection of acoustic phonons. (See, e.g., E. Schäffer, PhD Thesis, Univ. of Konstanz, Germany (2001), the disclosure of which is incorporated herein.) Their reasoning relied on a phenomenological model in which the internal radiation pressure in the polymer film was greatest beneath the areas of the polymer films experiencing protrusions. In this way, the destabilizing radiation pressure beneath the protrusions can exceed the stabilizing capillary pressure due to surface tension and these regions of the film will grow toward the top colder plate at the expense of the indentations from which mass is being removed.
The thermocapillary growth method described here differs significantly from the acoustic phonon growth method described by Schäffer et al. and includes a number of advantages. First, unlike the acoustic phonon method, the shaping process does not depend on the degree of reflectivity of the interfaces present, which may include liquid/solid, liquid/liquid or liquid/gas interfaces. Also, the thermocapillary technique does not depend on the speed of sound in the polymer film, which changes with temperature, pressure and the molecular weight of the polymer used. In fact, the thermocapillary growth method will work with any material that can be liquefied, not just polymer based films. And since the technique relies on film patterning by thermal gradients i.e. spatial and temporal variations of the temperature field, the actual value of the temperature fields used is not a constraining factor; that is, the values of the temperatures used for patterning can be suited to the material at hand since only thermal gradients are important to the shaping process. Secondly, for a given liquefied film subject to a specified temperature gradient, the thermocapillary technique is able to generate feature sizes smaller by a factor (ho/do)1/2, where ho is the thickness of the initial liquefied film and do is the distance separating the warm and cooler substrates in the example shown in FIG. 1. Thirdly, film processing does not require flat or parallel substrates nor contact with an opposing substrate. Knowledge of the thermocapillary mechanism leading to pattern formation allows the operator to use a wide spectrum of supporting substrates (including those with curvature or substrates transparent to illumination or radiation) and allows process intervention at any time, either to stop the growth of structures which have achieved the desired height, spacing or shape, to refine the shaping process during growth, or to redirect the flow toward other directions during shaping. In addition, all the working parameters necessary for constructing shapes of a desired form are known material or geometric quantities, whereas the acoustic phonon model depends on a phenomenological parameter, Q or Q bar, called the acoustic quality factor, which is not known. This parameter must instead be obtained from a fitting procedure extracted for this purpose from additional experiments. Furthermore, the material that constitutes the film to be patterned does not need to undergo any change in its chemical properties as required by photolithographic patterning methods. In addition, patterns can be produced without chemical etching or optical projection techniques, the latter of which creates a fundamental physical limitation to the lateral resolution of the pattern based on the wavelength of light used.
The thermocapillary lithographic method can be contrasted with photolithographic techniques, which rely on photoinduced crosslinking or other chemical processes in order to distinguish those parts of the exposed film that can be removed or retained. Moreover, in the thermocapillary growth technique, the lateral resolution of the pattern can be actively controlled by the particular value (e.g. spatial and temporal) of the local applied thermal gradient, the material properties of the liquefied film and gas or liquid overlayer, or geometric parameters like ho or do. The value of the local applied thermal gradient can also be tuned actively and in situ, thereby allowing for formation of disparate feature sizes in one process step
Unlike the acoustic phonon method, embossing methods or other conventional printing techniques, there is no requirement that the film to be patterned come into mechanical contact with the pattern template. Specifically, because the patterned structure grows by mass transfer from within the underlying film, and because this growth is triggered by the applied temperature gradient, there is no need for the structure to touch the pattern template at all. This is advantageous because physical contact with a mask or pattern can lead to complications in image separation.
Despite the intense interest in the use of acoustic phonons methods for film patterning, thus far no one has been able to manufacture working micro and nanoscale devices. Nor have the theories put forward thus far been able to adequately model the growth phenomenon such that controlled growth patterns can be formed. Accordingly, a need exists for a method and apparatus for the fabrication of 2D and 3D structures from liquefied films exposed to temperature gradients that will allow for the creation of predictable and ordered micro and nanoscale structures.